1. Field of the Invention
The present invention is generally related to cable actuated switches and, more specifically, to a cable actuated switching mechanism that is able to lock the mechanism in place upon actuation and actuate switch contacts also lock the mechanism in place in response to a broken cable.
2. Description of the Prior Art
Many types of cable actuated switches are known to those skilled in the art. Cable actuated switches are typically used in applications where an emergency stop capability is required along an extended distance. For example, in certain conveyor system applications, it is necessary to provide a means for operators to actuate the emergency stop condition from many different locations along the conveyor. Rather than provide numerous emergency stop switches at multiple locations along the equipment, it is economically advantageous to provide a single switch that can be actuated by pulling a cable that extends along the conveyor from the switch to a remote location.
U.S. Pat. No. 3,870,846, which was issued to Filip on Mar. 11, 1975, discloses a cable activated switch that comprises a switch support body that has a through bore. A first switch contact member is retained on the body and a second switch contact member is further slidingly retained on the body and insulated therefrom. Clamping means are provided for securing the cable passing through the bore. First resilient means are provided to bias the contact members. The first and second contact members are displaced relative to each other by predetermined axial movement of the cable which passes through the support body.
U.S. Pat. No. 5,003,135, which issued to Piccoli on Mar. 26, 1991, describes a cable controlled electrical safety switch device that comprises a piston tensioning cable under the action of a spring via a rod and a screw thread for adjusting the tension of the spring and of the cable. A piston groove actuates a push member for the switch. The piston is angularly adjustable. The flank of the groove remote from the spring is helicoidal. When the cable is long, a high tension is selected so that the groove flank moves away from the push member. This distancing is desirable in order that any length variations due to heat, which are greater with a long cable, may be prevented from triggering the switch. The clearance between the other flank and the push member is then corrected by rotation of the piston.
U.S. Pat. No. 4,396,815, which issued to Kobayashi et al on Aug. 2, 1983, discloses an emergency switch for preventing an accident in a mechanism employing a control cable. It comprises a casing having a pair of contacts at opposite inner side surfaces thereof and an insulator member having a movable contact. The insulator member is slideably and axially moved within the casing in connection with tensile force of inner cables. When the inner cables become unoperable because of some problem, the movable contact is touched to the contacts provided on the inner side surfaces of the casing in order to detect the problem or to stop the movement of the mechanism.
U.S. Pat. No. 3,956,606, which issued to Reiter on May 11, 1976, describes a cable operated safety stop switch. The switch unit, which is suitable for use in instances of emergency and also for a normal electrical shut off and resetting of a controlled system, features a snap action electrical switch. The switch can be operated selectively by a pair of like end anchored tension cables which have their inner ends connected to the operating and reset signal arm in order to trip the latter from a normal release position upon a tensioning of either cable by an attendant. The tripping of the arm causes it to operate the snap action switch and the arm is automatically locked in the tripped position thereof. A tensioning of either one of the cables under a force exceeding a very moderate value occasions a limited rotation of a shaft carrying the arm. This actuates the snap action switch and thereby through conventional wiring means initiates an instantaneous cut off of the system's electrical supply. The shaft and the arm are automatically locked in their tripped condition by a locking plate fixed on and rotatable with the shaft. The plate presents locking pins adapted to engage the latch in a fixed keeper plate of the switch unit. The locking plate is axially movable with the shaft in opposition to relatively mild spring bias to disengage the locking pins from the keeper plate. It thereby releases the shaft for normal counter rotation from locked condition to normal release condition.
Several problems are common with known cable actuated switch mechanisms. For example, if the switch actuation mechanism does not provide a mechanical snap action, or locking capability, a slight tension placed on the cable can momentarily change the status of the associated switch, but then return the switch to its original status when the tension on the cable is released. In certain circumstances, the switch associated with the mechanism may not be electrically connected in such a way that it automatically turns off all associated devices in response to a momentary deactuation of the switch. In this event, the release of the cable can then reactivate the associated machinery and cause physical harm to the operator. It would therefore be significantly advantageous if a mechanism could be provided that mechanically locks the mechanism in a deactivated condition as soon as the switch is deactivated and, in turn, require a positive action on the part of the operator to reset the switch following this type of circumstance.
Another problem that can occur with cable actuated switching mechanisms is the occurrence of a broken cable. When a cable breaks, an operator is unable to activate an emergency stop switch when a subsequent emergency occurs. It would therefore be advantageous if a switch mechanism could be devised in such a way that a broken cable situation causes a response which is similar to the mechanism's response to an emergency stop situation.
When long cable lengths are used in association with a cable actuated switch, changes in temperature can activate or deactivate the switch because of the resultant changes in the length of the cable as a result of the cable's thermal coefficient of expansion. With regard to the expansion or contraction of the cable as a result of temperature change, it is much more common for the cable to experience temperatures that are much higher than to experience those when the cable was initially installed than the opposite condition caused by falling temperatures. This occurs because many applications of cable actuated switches are used in circumstances, such as warehouses, where heating is provided for winter conditions, but air conditioning is not provided for summer conditions. As a result, heating systems are able to maintain the apparatus at normal operating temperatures during winter months, but no air conditioning systems are provided to maintain the apparatus at normal operating temperatures during summer months. As a result, the cables can expand beyond their normal lengths during summer months.
It would therefore be significantly beneficial if a cable actuated switching mechanism could be provided which addresses these problems.